llext: automatically merge sections by type

This patch changes the way sections are mapped to memories. Instead of
looking at the section name, each section in the ELF file is mapped to
the llext_mem enum by looking at the section type and flags.

This allows for a more generic mapping that works for both the ARM and
Xtensa cases, and also allows for sections to be merged if they are
contiguous and non-overlapping in the ELF file.

This patch also fixes a number of corner cases, such as in the logging
test where a section with read-only data was being ignored (not copied
and not relinked).

Signed-off-by: Luca Burelli <l.burelli@arduino.cc>